1. Field of the Invention
The present invention relates to an optical head including a light source emitting a light beam, an objective lens for projecting at least one light beam originating from said light beam emitted by said light source onto an optical record medium as a light spot, information being recorded on a record plane of said optical record medium, and a photodetecting means for receiving at least one light beam reflected by the optical record medium to detect the information recorded on the optical record medium and information representing a positional deviation of the light spot with respect to the record plane of the optical record medium.
2. Description of the Related Art
There have been proposed various types of the optical heads. For instance, in Japanese Patent Application Laid-open Publication Kokai Hei 2-301029, there is disclosed a known optical head. FIG. 1 shows this known optical head which is used for an apparatus for reading and writing data on and from a magneto-optical record disk. In this known optical head, a laser beam emitted from a laser diode 1 is made incident upon a polarizing beam splitter 4 by means of collimator lens 3 and beam shaping prism 19. The laser beam transmitted through the polarizing beam splitter 4 is focused onto a record layer 2a of a magneto-optical disk 2 by means of an objective lens 5. The laser beam reflected by the magneto-optical record disk 2 is made incident again upon the polarizing beam splitter 4 by means of the objective lens 5. Then, the laser beam is reflected by the polarizing beam splitter 5 is made incident upon a beam splitter 7 and is divided into two return laser beams.
One of the two laser beams reflected by the beam splitter 7 is made incident upon a first photodetector 13 having four divided light receiving regions by means of a lens system 12 including a cylindrical lens for introducing astigmatism. An output signal of the first photodetector 13 is processed by a first calculating circuit 16 to derive focusing error and tracking error signals. The other laser beam transmitted through the beam splitter 7 is made incident upon a polarizing beam splitter 9 by means of a half-wavelength plate 8. Then polarized components of the laser beam are divided and these polarized components are detected by second and third photodetectors 11 and 15 by means of condenser lenses 10 and 14, respectively. Output signals of the second and third photodetectors 11 and 15 are processed by a second calculating circuit 17 to derive an information signal representing data recorded on the record layer 2a of the magneto-optical record disk 2. There is arranged a magnetic field forming means 6 on a side of the record disk 2 which is opposite to the objective lens 5.
In the specification of this application, the optical path from the light source 1 to the record medium 2 is called a forward optical path and a light beam propagating along this forward optical path is termed as a forward light beam. The laser beam reflected by the record medium is also called a return laser beam and the optical path from the optical record medium to the photodetector along which the return laser beam propagates is sometimes termed as a return optical path. In the optical head to which the present invention belongs, a part of the forward optical path is identical with a part of the return optical path. In the optical head shown in FIG. 1, an optical path from the polarizing beam splitter 4 to the optical record medium 2 is used as the common optical path for both the forward light beam and return light beam.
In the optical head, in order to detect the return light beam, the return light beam has to be separated from the forward light beam, because the photodetector could not be arranged at the same position as the light source. In the known optical head shown in FIG. 1, the return laser beam emanating from the objective lens 5 is separated from the forward laser beam by means of the polarizing beam splitter 4. That is to say, the return laser beam is reflected by the polarizing beam splitter into a direction which is at right angles to a direction in which the forward laser beam is made incident upon the polarizing beam splitter. Therefore, the optical system of the optical head is liable to be large in size. Furthermore, the laser diode 1 and the photodetectors 11, 13 and 15 could not be provided on the same substrate or member, and thus the number of parts is increased and a reliability of the optical head might be decreased due to the temperature variation.